First, let me state that I have played Civ 1, 2 & 3, and Aplha Centauri, plus the demo of 4. The demo of 4 was not on my computer, and eventhough I liked it, I can't play it on mine as my graphics card isn't strong enough.
Second, the step towards 3D has hurt many games, though that's not an issue for the civ's I play. But for other games it is. Someone mentioned C&C generals as a bad example for the "3D gameplay devolution". In fact, the problem was not the step towards 3D, but EA. Moreover, their rushing its production. Try to play that game with the Shockwave mod (you need Zero Hour too), and you'll notice that this game is actually quite good.
Now, how I see things... Eventhough I love civ 1 & colon. to bits, those dosgraphics are rather harsh. Also, it was possible to rule the world with just 1 chariot.
The barbarians were cool as they kept comin & evolving, and could even occupy cities. I miss that a bit in III.
What do I miss more from I? The empire split when you occupy a capital. The diplomats as units (they were even good for popping huts... or a nice income-boost when you killed a hostile one) And off-course, the fact that you could improve science how you wanted, not bound by periods of mandatory research.
CivII I played later as civIII, and is in my opinion the lesser of the 3. I don't know why exactly...
Now, for the big comparison: pitting Alpha Centauri against CivIII.
Graphicwise, they're on par.
Science research is clunky with both. Smac's lack of a visual techtree does hurt it a bit. But once you get to something, then Sac shines in its true virtue- customisation.
On the other hand CivIII has tribe-specific units and ARMIES
But again, Smac takes the lead with covert-ops and diplomacy. In that game, the other factions do acknowledge your past good deeds in a way, while in CivIII the ai-players only seem to remember the bad things...even if you have not wronged them.
And the cultural-border war thingie is both a blessing & a curse in CivIII. Especially because it costs a heck of a lot of money to keep them. And that's 1 thing I always seem to lack

With other TBS games, I seldom have that problem...)
CivIII, while a very good game imho, does loose from smac for a first position. But it does have a solid 2nd place
